Power source-driven machines such as, for example, excavators, dozers, loaders, motor graders, and other types of heavy equipment typically include a cooling system that cools the associated power source and/or other machine components below a threshold that provides for longevity of the machines. The cooling system may include one or more air-to-air and/or liquid-to-air heat exchangers that may chill coolant circulated through the power source and/or machine components, and/or combustion air directed into the power source. Heat from the coolant or combustion air is transferred to air from a fan that is speed controlled based on a temperature of the power source, the temperature of machine component(s), and/or based on a temperature of an associated hydraulic system. The fan may also aid in circulating air in a machine compartment or space to increase the rate of heat dissipation.
The cooling system fan may be hydraulically powered. That is, a pump driven by the power source draws in low-pressure fluid and discharges the fluid at elevated pressures to a motor that is connected to the fan. When a temperature of the power source, machine component(s), and/or machine space is higher than desired, the pump and motor may work together to increase the speed of the fan. When the temperature of the power source, machine component(s), and/or machine space is low, the pump and motor work together to decrease the speed of the fan and, in some situations, even stop the fan altogether. Under some conditions, fan rotation can even be reversed such that airflow through a heat exchanger is also reversed to help dislodge any debris that has collected in the heat exchanger.
In some machine operating conditions, a hydraulic circuit driving the cooling fan described above and/or other hydraulic circuits of the same machine may have excess energy capacity and may store at least a part of this excess energy capacity in one or more accumulators. Energy from one or more of the accumulators may later be used to supplement prime mover, engine, and or other energy producing or storing devices.
An energy management system may be used to ensure that machine power is sufficient to meet the needs of all machine components and to release stored power when needed. The energy management system may monitor and control the storage and release of energy from one or more hydraulic accumulators associated with a hydraulic fan circuit to provide needed power to machine components based at least partially on an estimate of the fluid charge of the one or more hydraulic accumulators.
US Patent Application Publication US20080174174 A1 filed by Burns et al. discloses that the amount of energy stored in an accumulator is a function of the accumulator pressure and the volume of fluid stored in the accumulator. The temperature of the system, the type of gas used to pre-charge the system, and the initial pressure of the pre-charge gas can impact the amount of energy stored at a given accumulator pressure. The equation to calculate the energy stored in an accumulator is: E=(Pc*Vc−(P*Vc*((Pc/P)^(⊥/k))))/(1−k); where: E is the energy stored in the accumulator; Pc is the pre-charge pressure of the accumulator; Vc is the volume of gas in the accumulator at pre-charge; P is the current accumulator pressure; and k is ratio of specific heats (Boltzmann constant) for the pre-charge gas. The value of k for a gas varies with pressure at high pressures. Values of 1.3 to 1.8 may be used for typical gases and pressures. The pre-charge gas, pre-charge pressure, and volume of gas in the accumulator will not vary on a trailer during operation. Thus, the State Of Charge (SOC) of a hydraulic accumulator is a function only of its pressure. Although the accumulator pressure will vary with charge gas temperature, the SOC can be determined with acceptable accuracy even if this term is ignored.